Engine-driven machines such as, for example, dozers, loaders, excavators, motor graders, and other types of heavy equipment typically include a cooling system that cools the associated engine and other machine components below a threshold that provides for longevity of the machines. The cooling system typically consists of one or more air-to-air or liquid-to-air heat exchangers that reduce a temperature of the coolant circulated throughout the engine or combustion air directed into the engine. Heat from the coolant or combustion air is passed to air from a fan that is speed-controlled based on a temperature of the engine and/or associated hydraulic system.
The cooling system fan is generally hydraulically powered. That is, a pump driven by the engine draws in low-pressure fluid and discharges the fluid at elevated pressures to a motor that is connected to the fan. When a temperature of the engine is higher than desired, the pump and motor work together to increase the speed of the fan. When the temperature of the engine is low, the pump and motor work together to decrease the speed of the fan and, in some situations, even stop the fan altogether. Under some conditions, the fan rotation can even be reversed such that airflow through the heat exchanger is also reversed to help dislodge debris that has collected in the heat exchanger. In addition, since the fan and motor may function as a parasitic load on the engine, some systems may utilize the fan and motor to assist in reducing engine speed (i.e., braking).
Although effective at cooling the engine and reducing engine speed, it has been found that the hydraulic circuit driving the cooling fan described above may have excess capacity at times that is not utilized. With increasing focus on the environment, particularly on machine fuel consumption, it has become increasingly important to fully utilize all resources.
One attempt to improve hydraulic circuit efficiency is described in U.S. Pat. No. 7,658,065 that issued to Smith et al. on Feb. 9, 2010 (“the '065 patent”). Specifically, the '065 patent describes a hydraulic circuit having a hydraulic actuator, a primary pump configured to draw low-pressure fluid from a sump and discharge pressurized fluid to the actuator, and an in-sump energy recovery device configured to receive waste fluid returning from the actuator to the sump. The energy recovery device is driven by the returning waste fluid to feed low-pressure charge fluid to the primary pump, thereby reducing work performed by the primary pump. A flywheel is connected to the energy recovery device to kinetically store energy extracted from the returning waste fluid and selectively supply the stored energy back to the energy recovery device.
Although the energy recovery device of the '065 patent may improve efficiency of the associated hydraulic system, it may increase the cost of the system and have limited applicability. That is, the energy recovery device is an additional component that only performs energy recovery functions. Specialized devices like the energy recovery device add cost to the system. In addition, the energy recovery device is only shown and described with respect to an open circuit system. The disclosed recovery device is not particularly well suited to assist in reducing engine speed, nor is it easily calibrated without experienced technicians.
The disclosed hydraulic fan circuit is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.